1. Field of the Invention
The present invention relates to an X-ray imaging apparatus for performing X-ray imaging of the oral cavity or other portions of the human body, for detecting X-ray images by using an X-ray imaging device, such as a CCD (charge-coupled device), sensor, a MOS (metal-oxide semiconductor) sensor or an ionization chamber and for displaying the X-ray images, as well as to an automatic density correction method for the apparatus.
2. Description of the Related Art
FIG. 5 is a block diagram showing an embodiment of a conventional X-ray imaging apparatus. When X-rays are emitted from an X-ray generator 51 toward a subject OBJ, the X-ray image of the subject OBJ is detected by a CCD sensor 52 and converted into an electrical signal. At this time, the CCD sensor B2 outputs an analog signal proportional to the intensity of the received X-rays.
The analog signal is amplified by a predetermined amplification factor by a preliminary amplifier 53 of the next stage, and converted into a digital signal with a quantum of 8 or 10 bits, for example, by an AD (analog-digital) converter 54 provided at the further next stage. The digital image signal is then directly delivered to an image display unit 55 and displayed on a CRT (cathode-ray tube), a liquid crystal display panel or the like.
Since the image data obtained after AD conversion is directly displayed in the case of the conventional X-ray imaging apparatus, the data is displayed as a signal being linear to the intensity of X-rays.
FIGS. 6A, 6B and 6C are graphs, each showing the relationship between input data and display density after AD conversion. The abscissa indicates the input data "in" and the ordinate indicates the display density "out" of the image display unit 55. As the incoming X-ray dosage to the CCD sensor 51 increases, the signal level of the input data "in" becomes higher, and it is assumed that the display screen becomes blackish.
First, in FIG. 6A, although the relationship between the input data and the display density is linear, since the inclination of the linear portion is gentle, the display density is low and the display screen becomes whitish as a whole, indicating that the intensity of the X-rays having reached the CCD sensor 51 is insufficient.
In FIG. 6B, since the relationship between the input data and the display density is linear and the inclination of the linear line is approximately 45 degrees, both the display density and the X-ray intensity are proper.
In FIG. 6C, although the relationship between the input data and the display density is linear in the low dosage region, the display density is saturated to a pinch-black condition at a certain dosage or higher. The entire screen becomes blackish as a whole, indicating that the X-ray intensity is excessive.
When the intensity of X-rays having reached the CCD sensor 52 is excessive (over exposure) or insufficient (under exposure), the display density of the image display unit changes significantly, and the image cannot be used for diagnosis. Since the intensity of X-rays having reached the sensor changes greatly depending on the properties and thickness of the subject OBJ, it is considerably difficult to set the image signal level within a proper display density range. In addition, a repeated X-ray imaging increases radiation dosage, and should be avoided as much as possible. In particular, when the X-ray intensity is excessive during dental X-ray imaging, a saturated portion tends to be mistaken for dental caries, and there is a possibility of a wrong diagnosis.
As related art, Japanese Laid-open Patent Application No. Hei 5-95512 relates to an X-ray digital angiography apparatus, and discloses a method for detecting the maximum and minimum of the output data of an AD converter and for writing the contents of a look-up table. However, this method has the following problems: 1) Both the maximum and minimum of image data are necessary; 2) The method for actually creating the look-up table is not described; 3) The look-up table is not optimized; and 4) The method is applicable only to angiography using an X-ray TV.